Field of the Invention
The present invention relates to a separation membrane suitably used in purification of sewage, namely, domestic wastewater exhausted from life environments such as cooking, washing, taking a bath, and relieving nature, and of wastewater discharged from manufacturing plants, restaurants, fish processing factories, and food processing factories. The present invention also relates to a method for making the separation membrane. Moreover, the present invention relates to a separation membrane element, a separation membrane module, and a sewage treatment apparatus including the separation membrane.
Description of the Related Art
Separation membranes have recently been used for purification of sewage and wastewater. Though various types and shapes of separation membranes are known, a flat membrane called a microfiltration membrane attracts attention. The microfiltration membrane is generally formed as follows. A resin solution containing a pore-forming agent is applied onto a surface of a porous substrate such as woven or nonwoven fabric or is impregnated into the porous substrate, and the resin is coagulated to form a porous resin layer on the porous substrate. The porous resin layer functions as a separation layer. Unfortunately, the flat membrane does not have a large effective area per unit area, compared with other types of separation membranes, for example, a hollow fiber membrane; hence, the flat membrane is required to achieve high water permeability while maintaining a micropore size corresponding to the object to be filtered. When the porosity is increased in order to achieve high water permeability, the micropore size excessively increases or surface cracks causing a decrease in rejection occur. When the micropore size is decreased in order to achieve a high rejection, the water permeability inevitably decreases. Accordingly, a high rejection and high water permeability are basically incompatible. It is difficult to achieve balanced compatibility therebetween.
In addition, separation membranes for sewage water undergo heavy collision of solid materials such as sand and sludge in use and heavy collision of bubbles during an aeration process which is performed to supply oxygen into activated sludge and to prevent clogging. Thus, the separation membrane must have sufficiently high strength durable to such severe impacts. Such high strength is primarily borne by the porous substrate. In any known separation membrane, the porous resin layer would be separated from the porous substrate during a filtration process and an aeration process in severe cases.